The permeability of soil is defined as the soil's conductivity to fluid flow. The permeability of soil to fluid flow depends upon the magnitude of soil gas and groundwater flow when subjected to particular natural and/or unnatural pressure gradients. Pressure gradients exist due to natural effects such as hydraulic gradients (in the case of groundwater) and barometrically imposed gradients (in the case of soil gas). Unnatural (forced) gradients can be imposed by soil vapor extraction, air sparging, active venting, pump and treat, and other remediation processes requiring the movement of fluids through the soil.
The design of any of these processes requires a knowledge of the flow characteristics of the soil to be remediated. The soil's permeability is the largest variable, which can vary by orders of magnitude in any given hydrological and/or geological environment. Therefore, knowledge of soil gas permeability is required to design soil vapor extraction systems and understand, in general, the movement of gas in the soil. Similarly, knowledge of saturated hydraulic conductivity (or, the soil's permeability to liquid flow) is required to predict movement of groundwater in saturated soils.
Soil permeability has historically been measured either in laboratories on a very small scale or in the field on a very large scale. Laboratory measurements rarely agree with data collected in the field due to the difficulty of obtaining truly undisturbed soil samples. Further, laboratory test results are usually at least an order of magnitude lower than actual field results.
Because of the high cost and time constraints of obtaining field measurements, it is oftentimes beneficial to first obtain soil permeability measurements in a laboratory setting. The flow of fluid and the travel of contaminant plumes in subsurface soils are capable of being mathematically modeled if the soil's permeability is known. Frequently, however, it is difficult to readily determine the accuracy of the soil's permeability for several reasons. For example, soil is heterogeneous in varying degrees, usually depending upon the type of soil in the surrounding environment, the depth of the soil and the physical scale of interest. Additionally, it is known that soil permeability can vary between two to three orders of magnitude at most soil remediation sites. Consequently, the ability to obtain quality predictive modeling results in the laboratory, whether to estimate soil gas travel or to design alternative remediation systems, is heavily dependant upon the accuracy of the predicted soil permeability and the surrounding environment.
In the field, soil gas permeability measurements are obtained either through total borehole flow or isolated packer (also referred to as a "straddle packer") measurement techniques. Total borehole flow measurements are obtained from open or screened boreholes, where gas or liquid is injected into or extracted from the borehole well. In particular, permeability measurements (gas or liquid) are typically obtained from boreholes using a cylindrical flow model and geometry. Long screened or uncased sections of the borehole are subjected to unnatural (e.g., forced) pressure gradients and the resultant flow into or out of the well is subsequently measured in order to obtain the soil permeability. For one-dimensional radial symmetric (cylindrical) flow geometries such as these, the test region is relatively long and a radius of influence is either measured (or can be predicted) to determine the surrounding soil's permeability. The inherent weakness with this approach, however, is that it results in providing only an average permeability over the test region, and cannot delineate stratigraphic features within any particular test region or depth.
A disadvantage to the current method of obtaining permeability measurements in the field is that it is impossible to translate unmodified open borehole measurement techniques to penetrometer measurement because of size limitations and the penetrometer's compaction of the soil.
Various direct push measurement techniques exist, with perhaps the use of penetrometer rods (or, "penetrometers") being the most common. The direct push technologies using penetration rods include an elongated rod which is pushed into the ground to penetrate the ground and subsurface depths. Generally, each penetrometer rod is a continuously cylindrical steel tube having a hollow interior channel. At one end of some penetrometer rods (e.g., the end which is embedded in the ground) is placed a cone-shaped tip (seen generally in FIG. 3). These types of penetrometers are referred to as "cone penetrometers." If desired, the penetrometer rod can travel deeply into the subsurface by the assistance of a hydraulic ram or other conventional means.
Use of a penetrometer rod to obtain permeability data is inherently less intrusive than drilling boreholes. Penetrometers provide vastly more data in the same amount of time as do drilled holes, at a much lower cost and risk to the operators of penetrometer. Penetrometers, and other direct push techniques (such as the ResonantSonic system) are rapidly advancing as hole formation and soil characterization tools because they are capable of emplacement in difficult media. Therefore, conducting permeability measurements with direct push techniques, instead of in drilled boreholes, retains all of the advantages of penetrometer emplacements.
Conventional cone penetrometer systems are already outfitted for soil gas and liquid sampling, geophysical measurements, in-situ chemical analysis, temperature logging, pore pressure measurements, and direction indicating capabilities. For example, permeability measurements are conducted with cone penetrometer emplacements by observing the dissipation of pore pressure after the soil has been compacted by the rod emplacement. The ability to obtain pore pressure data is included in a conventional geophysical measurement package located at the tip of the cone penetrometer. A disadvantage to this type of testing, however, is that this type of measurement requires a knowledge of the soil type to infer the soil's permeability, which in many cases is difficult to predict. Furthermore, this type of testing cannot be conducted in high permeability zones because the pressure dissipation in the soil is too rapid.
Conversely, conducting cone penetrometer testing using a spherical flow model, as described in the present invention, can provide detailed soil permeability data as a function of the depth at which the measurement is taken. This is because the testing region is relatively small (measured in fractions of a meter versus meters for the cylindrical model), allowing discrete measurements at high resolution in boreholes.
Therefore, it is an object of the present invention to provide a measurement method which allows quantitative in-situ determination of gas and saturated liquid permeability with a modified cone penetrometer and other direct push techniques.
It is also an object of the present invention to provide a soil permeability measurement method which substantially reduces field costs, is rapidly emplaced, generates minimal secondary waste generation and reduces worker exposure to chemical and radiological hazards.
It is a further object of this invention to obtain steady state measurements of air and saturated liquid permeability at various subsurface depths during a direct push technique which is unaffected by the compaction of the soil caused by the penetrometer.
It is also an object of the present invention to utilize a spherical flow geometry measurement method, in conjunction with direct push techniques, to obtain information relating to soil permeability as a function of depth.
It is another object of the present invention to provide a in situ measurement apparatus adapted to employ a spherical flow model to obtain information relating to soil permeability as a function of depth, without substantial disturbance of subsurface soil.